Motor vehicles which are supported by inflatable tires generally have a desired inflation pressure. The improper inflation of a tire can lead to poor gas mileage, increased tire wear and to a loss of handling ability, particularly during emergency braking and maneuvering conditions.
Unfortunately, many motor vehicle operators do not take the time to monitor the pressure in their tires. Once the tires are inflated, pressure generally is not measured unless a tire is visibly under-inflated or tire inflation has caused a defect in vehicle handling which is apparent to the operator. By this point, the tire has already been operated in a state of improper inflation.
Prior art devices have attempted to resolve this problem by providing an automatic tire pressure monitoring system for monitoring the pressure within the tires and providing an indication to the vehicle operator when one or more of the tires reaches a condition of improper inflation. One class of such tire pressure monitoring systems includes a tire pressure sensing apparatus as well as a transmitter contained within each of the tires. The tire pressure is monitored and transmitted to a central receiving unit which in turn provides an indication to the vehicle operator.
The use of airwave communications between the transmitter and receiver creates a host of problems. With the transmitting unit wholly contained within a particular tire, relatively weak signals are transmitted since the transmitter must rely upon a battery as a source of power. Further, the metallic configuration of a vehicle frame and body is not generally conducive to the transmission of signals from the wheel wells of a vehicle to a centrally located monitor. Further, as a wheel rotates, a transmitted tire pressure signal is subjected to a doppler shift based upon this rotation if the transmitter antenna is not oriented in a fashion such that its center of mass lies along the axis of rotation of the wheel.
The reliance upon a battery in these prior art systems provides a further problem, that of limited battery life. If a battery is installed in a tire pressure monitoring system within a tire, that battery will require replacement at some point which could require the dismounting of the vehicle tire from the wheel. An alternate source of power for a tire pressure monitoring system is presented by U.S. Pat. No. 4,160,234 to Karbo, et al. This patent discloses a self-powered tire condition indicator system in which a plurality of wheel mounted transmitting assemblies communicate by radio frequency transmission to a receiver located within the operator compartment of the vehicle. Each of the transmitting assemblies generates its own transmitting power by converting the mechanical compression of a flat tire into electrical energy using a piezo-electric transducer. This system, while not requiring a battery for operation, suffers from the fact that a tire must be in a severely deflated condition to energize the unit. Thus, this system is unable to provide a monitoring of the tire pressure during conditions when the tire is properly inflated or if the tire deflation is slight.
A further problem with prior art tire pressure monitoring systems where a pressure sensor and transmitter unit is contained within a tire is that reprogramming or testing of the unit requires dismounting of the tire. This is an inconvenient and expensive process.
Prior art tire pressure monitoring systems further include monitoring of the temperature within a tire. This is important given that increased tire temperature can be indicative of underinflation or other tire defects which again could be a source of concern for a vehicle operator. These prior art systems, however, lack the ability to compare the sensed tire temperature with an ambient temperature to determine if the tire temperature is abnormal given ambient conditions.